Sforno on Genesis 18:15/ספורנו על בראשית י״ח:ט״ו

1כי יראה, to say “I have sinned;” but she repented the lie in her heart.

2!ויאמר לא, for the angel knew that as Bileam said (Numbers 23,19) “seeing than man is not G’d, he lies and deceives,” לא איש א-ל ויכזב. He did not believe her at all.